1. Field of the Invention
This invention relates generally to a method for triggering an anode bleed from a fuel cell stack and, more particularly, to a method for triggering an anode bleed from split fuel cell stacks that includes triggering the bleed if an estimated concentration of nitrogen in the anode is greater than a predetermined threshold concentration, triggering the bleed if the absolute value of the difference between the average split stack cell voltages is greater than a predetermined threshold voltage, or triggering the bleed if the voltage spread of the fuel cell voltages in either of the split fuel cell stacks is greater than a predetermined threshold voltage.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Even though the anode side pressure may be slightly higher than the cathode side pressure, the cathode side partial pressures will cause air to permeate through the membrane. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases above a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode exhaust gas output of the fuel cell stack to remove nitrogen from the anode side of the stack.
It has been proposed in the art to provide stack order switching or anode flow-shifting in a fuel cell system that employs split stacks. Particularly, valves and plumbing in the system are provided so that the anode exhaust gas exiting a first sub-stack is sent to the anode of a second sub-stack, and the anode exhaust gas exiting the second sub-stack is sent to the anode of the first sub-stack in a cyclical manner.